Bearings and in particular rolling bearings are used in a wide range of applications comprising rotating shafts. Rolling bearings provide low friction rotation by means of the rolling elements arranged between the outer and inner race of the bearing. To further reduce the friction between the rolling elements and the races, thereby increasing the life length of the bearing, a bearing lubricant such as grease or oil is often used to lubricate the rolling elements.
In order to improve the availability/up-time and life span of, for example, a wind turbine, various bearing related parameters may be monitored, such as, vibrations, lubricant pressure, lubricant quality, lubricant temperature and bearing load.
It has been found that bearing failures are often caused by water in the bearing lubricant. Water in the lubricant can cause surface erosion and cavitations within the bearing. Also, it can cause hydrogen embrittlement due to the extreme pressures in rolling element bearings, which can be as high as 1 to 3 GPa, where water can break down into its constituent atoms causing hydrogen to penetrate the surface of the bearing elements and cause the surface to become more brittle. Water can also cause additives to precipitate and cause abrasive particles or sludge to form. Water also causes the base oil to oxidize much more quickly. Even without the addition of external water, water content in the lubricant may increase slightly due to oxidation of hydrocarbons in the lubricant.
For example, a bearing exposed to the elements (such as a wheel bearing on a train or car) is more susceptible to contamination, both by water and by other contaminating particles.
Accordingly, it is important to be able to monitor the condition of the bearing so that a bearing can be replaced before it breaks.
Although the monitoring that is currently described in the art helps the scheduling of maintenance and prediction of failures before they occur, and thereby may improve the availability of rotating systems, many such systems are both complicated and impractical to use in many applications.